In the conventional manufacture of products containing cellulose material, a mass of fibers, chips or other such cellulose-containing material along with a heat-hardenable binder, fillers, catalysts and other additives is deposited as a loose mat into a belt conveyor system. While on the belt, the loose mat is usually transported through a preprocessor station where the mat is subjected to initial contact pressure which densifies and dewaters the mat before the mat is delivered to a press reactor station. There, through the use of contact heat and pressure, the mat is finally brought to the desired caliper and hardened state by thermal fusion of the binder material with the cellulosic fibers and other constituents of the compressed mat. After leaving the press station and after having cooled to an appropriate temperature, the board may then be transported to one or more downstream finishing stations where the board surfaces may be smoothed, embossed, etc. to form the finished product.
While that standard process has been used for many years to make various utilitarian cellulose-containing products such as underflooring and siding for the building industry, that old process and the products made thereby have several drawbacks. More particularly, the process itself is relatively time-consuming and expensive due particularly to the required residence time of the mat at the press reactor station. That is, in order to achieve the desired densification and bonding between the cellulose fibers and the binder material without carbonizing or burning the mat, the temperature in the press reactor must be kept relatively low. This prolongs the setting of the binder material and the fibers comprising the mat.
Also, when carrying out the standard process with standard press reactor apparatus, a large volume of steam and volatiles is generated within the press reactor due to pressure and heat-induced chemical reactions between the various mat constituents which are necessary to produce the finished product. This results in a pressure buildup which is difficult to control so as to allow the process to continue. In fact, since there is no provision for venting the steam and reaction gases except at the periphery of the mat, to avoid a blowout, the press platens usually have to be opened for a brief period to allow these gases to escape from the surfaces of the mat. This interruption of the process and the consequent pressure and temperature changes inflicted on the mat affect the ongoing internal chemical reactions to the extent that the resultant board product may have voids, blisters and density variations which adversely affect the overall quality of the product.
Also, if the prior process is practiced to make a cellulosic product suitable for exterior use, a substantial amount of heat-hardenable resin or binder material must be used to give the finished product sufficient wet strength and stability to render the product water and weather-resistant. When a mat containing one of the usual resin and binder materials, e.g. unreaformaldehyde, is subjected to the heat and pressure of the press reactor, toxic and noxious fumes are emitted which present a distinct hazard to operating personnel and give rise to potential problems complying with OSHA standards. Furthermore, the product itself may emit such fumes in the field if subjected to sufficient heat, e.g. if it should catch fire. While this may not pose a problem if the board product is being used as a concrete form, for example, it could do so, if the product is used as underflooring in a house, for example.
In an effort to avoid many of the aforesaid difficulties inherent in the standard cellulosic product-making processes and in the products themselves, I devised a process of permanently fusing the fibers and particles of such cellulosic products under pressure, temperature and atmospheric conditions that produces a new state of fusion and chemical combining of the cellulosic fibers and particles. This technique reduces the time required to make the product, and it produces a product which is relatively strong, water and weather resistant, and yet requires only a fairly small amount of resin or binder material.
In accordance with this process, which is disclosed in my U.S. Pat. No. 4,111,744, the cellulose-containing material, including any additives such as binder, fillers, catalysts, synthetic fibers, etc., having an equilibrium moisture content in the range of 2% to 50%, is introduced as a mat into an oxygen-excluding reaction station. In that station, the mat is positioned between press dies or platens having a controlled temperature in the range of 450.degree. F. to 800.degree. F. Also, to internally heat the mat, supplemental heat in the form of RF energy is applied to the mat at an intensity level depending upon the nature of the cellulosic materials and the rate of reaction desired. In some cases, the application of the RF heating is delayed with the mat being held at less than full die pressure to commence scavenging the mat of air and volatiles and to preheat the mat before the supplemental energy is applied.
As described in that patent, the ambient temperature to which the fibrous mat is subjected is well beyond the normal carbonizing temperature of cellulose, i.e. about 400.degree. F. However, the temperature of the mat is controlled in the oxygen-free atmosphere of the reaction station by microporous sheets that contact the opposite faces of the mat and are vented to the outside so as to permit the reaction process to continue without gas blowout, while keeping the carbonization of the mat to a minimum. As the platens close, the mat becomes fully consolidated to bring the mat to its final density and caliper, being heated all the time by the platens and RF source until the platens are opened to release the mat.
Then, as quickly as possible, the partially fused mat is transferred to an oxygen-excluding hot stacking station where a continuation of the fusion reaction is carried out under controlled temperature conditions. During the dwell time of the mat in the hot stacking station, which is substantially longer than the exposure time in the reaction station, the temperature of the mat is reduced gradually until the final product can be released from that station to the atmosphere at a temperature that enables the product to be handled or conveyed to one or more downstream finishing stations.
While the cellulose products made by my prior process are superior to those produced by the standard method described at the outset in terms of strength, stability, uniformity and weather resistance, there has been some difficulty in controlling the process carried out in the reaction station to avoid at least some discoloration and carbonizing of the finished product. The carbonizing is moderate and, to a large extent, confined to the surfaces of the product so that it does not materially affect the structural integrity of the product. However, it does adversely affect the appearance of the product, and therefore, is undesirable from a marketing standpoint if for no other reason.
My prior patented process is disadvantaged also in that it does require the presence of an oxygen-excluding stacking station immediately downstream from the reaction station to which the consolidated and partially fused mat must be transferred immediately to avoid total carbonizing or burning of the mat. Not only does the requirement for the stacking station increase capital and operating costs, but also, inevitably, at least some atmospheric oxygen reaches the hot mat during its transfer from the press reactor into the stacker giving rise to at least some carbonizing of the product. In addition, if the product is one which does include at least some binder material, product outgassing at the time of transfer can include toxic binder reaction products that can pose a hazard to workers in the vicinity of the process line.
Finally, the products resulting from my prior process, aside from being discolored, do have some variations in their internal compositions and densities apparently due to the fact that the chemical reactions occurring within the mats during the fusion reaction process are not uniform throughout the mats. Also, in some cases, their surface finishes are not as smooth as might be desired because of unwanted embossing of the mats by relatively large holes in the microporous sheets or plates that contact the mats during the reaction process.